1. Field of the Invention
This invention relates to an evaporative fuel-processing system for internal combustion engines installed in vehicles, and more particularly to an evaporative fuel-processing system which has a function of detecting abnormalities in an evaporative emission control system wherein evaporative fuel generated in a fuel tank of the engine is purged to an intake system thereof.
2. Prior Art
Conventionally, there has been widely used an evaporative fuel-processing system for internal combustion engines installed in automotive vehicles, which comprises an evaporative emission control system (hereinafter referred to as "the emission control system") having a canister provided with an air inlet port, a first control valve arranged across an evaporative fuel guiding passage extending between the canister and the fuel tank of the engine, and a second control valve arranged across a purging passage extending between the canister and an intake system of the engine.
An evaporative fuel-processing system of this kind temporarily stores evaporative fuel in the canister, and then purges the evaporative fuel into the intake system of the engine.
Whether or not an evaporative fuel-processing system of this kind is normally operating can be checked, for example, by forcibly bringing the emission control system into a predetermined negatively pressurized state, and then measuring a change in pressure within the fuel tank hereinafter referred to as "the tank internal pressure") occurring with the lapse of time after the system has been brought into the predetermined negatively pressurized state to thereby determine abnormality of the evaporative fuel-processing system. In this connection, reference is made to Japanese Provisional Patent Publication Kokai) No. 5-79408 and its corresponding U.S. Ser. No. 07/942,875 assigned to the assignee of the present application, in which is proposed an abnormality determining method of this kind.
More specifically, according to the method of the publication, there are successively carried out (1) an open-to-atmosphere process of the emission control system, which relieves the emission control system to the atmosphere, (2) a check of a change in tank internal pressure, which measures a rate of change in the tank internal pressure while the fuel tank is closed, (3) a process of reducing tank internal pressure, which negatively pressurizes the emission control system to a desired pressure value by the use of negative pressure from the intake system of the engine, and (4) a leak down check, which checks pressure recovering from the desired negative pressure to thereby determine whether or not leakage has occurred in the emission control system.
Further, in the system of the above-mentioned publication, a correction processing is carried out in order to prevent any misjudgment on leak check ascribable to various operating conditions of the fuel tank.
Specifically, if in bringing the emission control system into a negatively pressurized state by reducing the tank internal pressure as mentioned above, a rate of decrease in the pressure varies depending upon various operating conditions of the fuel tank, e.g. a fuel amount within the fuel tank, fuel temperature, and tank internal pressure under an open-to-atmosphere condition, so that a time period over which the tank internal pressure reaches a predetermined abnormality determination value varies, and hence the accuracy of abnormality determination is degraded.
More specifically, when the fuel tank is almost filled with fuel, the spatial volume at an upper portion of the fuel tank is small to increase the decrease rate of the tank internal pressure, whereas, when the amount of fuel within the fuel tank is small, the decrease rate of the pressure is low. Accordingly, there is the danger of a misjudgment, depending on the fuel amount within the fuel tank. Further, if the negative pressurization requires a long time period to complete, the leak down check also requires a long time period to complete as well. Therefore, it is necessary to correct the time period over which the negative pressurization is to be carried out. Still further, a high fuel temperature causes generation of a large amount of evaporative fuel within the fuel tank, resulting in a low rate of decrease in the tank internal pressure. Thus, there is also the danger of a misjudgment. Moreover, when the tank internal pressure is high under the open-to-atmosphere condition, if evaporative fuel is leaked during the negative pressurization, it takes a long time period for the tank internal pressure to lower to the predetermined abnormality determination value. Thus, there is also the danger of a misjudgment on the abnormality.
To avoid the above misjudgments, according to the method of the publication, the time period over which the tank internal pressure reaches the abnormality determination value is corrected according to various conditions of the fuel tank.
Further, to improve the publication method, another method has been proposed by the present assignee in Japanese Patent Application No. 3-360629 and its corresponding U.S. Ser. No. 07/942,875, wherein it is determined whether or not the emission control system is abnormal when a predetermined time period has elapsed during negative pressurization, i.e. before the latter is completed. According to this method, even when the emission control system cannot be brought into a predetermined negatively pressurized state during the negative pressurization due to a perforation in the fuel tank or the like, the abnormality determination can be carried out.
However, the above-mentioned conventional evaporative fuel-processing systems have the following disadvantages:
First, when the amount of evaporative fuel generated in the fuel tank is extremely large, as in the case where the fuel tank is placed under a high temperature condition for a long time period, so that it exceeds a limit value for correction of the above-mentioned time period, the correction cannot be effective to thereby cause a misjudgment.
Secondly, under such a high temperature condition where evaporative fuel is easily generated, a large amount of evaporative fuel is stored in the canister, and consequently, the evaporative fuel is directly drawn into the intake system of the engine during the negative pressurization of the fuel tank, resulting in extreme enrichment of an air-fuel mixture supplied to the engine, and hence an adverse effect being exerted on the driveability and exhaust emission characteristics of the engine. Furthermore, in such a case, even if the negative pressurization of the tank is continued, the pressure cannot be reduced to a desired value due to an increased air flow resistance of the canister which acts against a drawing force caused by vacuum in the intake passage. Therefore, there has been the danger of a misjudgment that leakage has occurred, or the determination has to be suspended.